The invention is directed towards the field of optical sensors and, more specifically, towards flattening the illumination intensity across an optical sensor.
The optical mouse is a commonly used input device in computer systems. The optical mouse has an optical sensor, a light source for illuminating a surface, and a lens for focusing light reflected from the surface onto the optical sensor. As the optical mouse is moved relative to the surface, it acquires a series of images of the surface. The optical mouse determines its own position relative to the surface by comparing the differences between consecutive images. Ideally, the illumination intensity of the light focused by the lens onto the optical sensor should be uniform, so that the acquired image accurately represents the surface.
However, it is well known in the art that the intensity of light off the axis from a lens is less than the light on axis by a factor of cos4xcex8, where xcex8 is the angle from the lens exit pupil to the off-axis point. (See chapter 6.7, FIG. 6.10 of Modern Optical Engineering, Third Edition by Warren J. Smith, McGraw-Hill, Inc. 2000 for more details). FIG. 1 illustrates the prior art rule of cos4xcex8 with a lens 101, the lens axis 103, an optical sensor 105, exit pupil 107 and the angle xcex8. The light transmitted to the optical sensor 105 at point B is less than the light transmitted to point A by a factor of cos4xcex8.
The rule of cos4xcex8 means that the light intensity across the optical sensor 105 will not be uniformly flat. Instead, areas of the optical sensor 105 that are closer to the lens axis 103 will experience greater light intensity than areas of the optical sensor 105 that are further from the lens axis 103. This variation in the light intensity across the optical sensor 105 may cause an optical mouse to misread the images it acquires of a surface.
An intensity flattener, such as a gradient neutral density filter, may be used to flatten the illumination intensity across the optical sensor. A gradient neutral density filter is simply a piece of glass with chromium or other light-blocking material sputtered onto it. The more chromium used, the more light that is blocked. The filter is placed either in front of or behind the lens 101 to flatten the illumination intensity. However, these filters are expensive, large, and difficult to position accurately, given the small size of the current optical sensors.
Therefore, there remains a need for a smaller and less expensive solution to flatten the intensity of light across the optical sensor.
In accordance with an illustrated preferred embodiment of the present invention, a method and apparatus for flattening the illumination intensity over an optical sensor is disclosed. Light-blocking patches are placed over the areas of the optical sensor where light intensity needs to be reduced. The goal is to achieve uniform intensity of illumination across the entire optical sensor. Areas of the optical sensor that fall closest to the axis of the lens receive the greatest intensity of light due to the rule of cos4xcex8, and therefore will require more light-blocking patches than areas of the optical sensor further away from the lens axis.
Although the light-blocking patches may block all wavelengths of light, it is not essential that they do so. Only the wavelength(s) of light used by the optical mouse need to be blocked. The light-blocking patches are added during the process of fabricating the optical sensor. An additional mask will be needed to deposit the light-blocking patches in the proper pattern.
Further features and advantages of the present invention, as well as the structure and operation of preferred embodiments of the present invention, are described in detail below with reference to the accompanying exemplary drawings. In the drawings, like reference numbers indicate identical or functionally similar elements.